Each year in the United States, 40 million tons of precious metal-bearing waste is generated. Of this, only about 5% is currently processed for recovery of the valuable constituents (Reddy, R. G., `Metal, Mineral Waste Processing and Secondary Recovery,, J. Metals, Apr. 1987, 34-38). Three main secondary sources of precious metals are aqueous solutions such as electroplating waste, solids and sludges such as salts and copper refinery anode slimes and metal scrap such as wire and printed circuit boards (Moore, J. J., Chemical Metallurgy, Butterworths, London, U. K., 1981, p. 267).
A method for silver recovery from secondary solid sources using a sulfuric acid leach was developed by Kunda (`Hydrometallurgical Process for Recovery of Silver from Silver Bearing Materials,` Hydrometallurgy, 1981, 77-97). His method for recovery of metallic silver from the sulfate solution involved precipitation of silver sulfate, dissolution of this silver sulfate and, finally, hydrogen precipitation of metallic silver.
Of the many electrolytic processes that have been commercialized for metal recovery from plating waste solutions, some are claimed to have applicability to precious metals. Examples include the Retec heavy metal recovery system which is based on electrolysis onto a porous metal electrode (Duffey, J. G., `Electrochemical Removal of Heavy Metals from Wastewater,` Products Finishing, Aug. 1983, 72-75) and the Andco heavy metal removal system, which is based on electrochemical precipitation (`Andco Heavy Metal Removal Systems, Actual Performance Results,` Andco Environmental Processes, Inc., not dated). Solvent extraction has also been investigated for recovery of silver from aqueous solutions (Rickelton, W. A. and A. J. Robertson, `The Selective Recovery of Silver by Solvent Extraction with Triisobutylphosphine Sulfide,` Society of Mining Engineers of AIME, Preprint No. 84-357, 1984).
Much of the technology for the recovery of precious metals from solids by smelting is derived from fire assaying techniques (Gold Institute, `The Fire Assay of Gold,` published by the Institute, Jan. 1985). Smelting for metal recovery is limited to materials of high precious metals content (generally greater then 10%) since the matrix containing the metals is destroyed by the process. Smelting would thus be limited to materials such as copper anode slimes and metal alloy scrap. Loaded sorbents or ion-exchange agents could be processed economically by smelting if they were very highly loaded, beyond the range of what is typical for carbonaceous or organic sorbents.
In general, smelting requires fluxes and other slag-forming agents. A reducing agent such as zinc or starch may also be required. The composition of the smelting charge must be determined on a case-by-case basis.
U.S. Pat. No. 4,456,391 (Reimann, `Recovery of Silver from Silver Zeolite`) discloses a process for recovering high purity silver from a silver exchanged zeolite (of unspecified composition) used to recover iodine from radioactive waste streams. The process involves heating the silver exchanged zeolite with slag forming agents to melt and fluidize the zeolite, releasing the silver. The silver concentrate is re-melted and treated with oxygen and a flux to remove impurities.
The toxicity of certain heavy metals such as lead has been known to man for centuries. In the United States, the Environmental Protection Agency has declared lead and its compounds to be priority environmental pollutants and has begun establishing concentration limits for drinking water(Tackett, S. L., `Lead in the environment: Effects of human exposure,` American Laboratory, Jul. 1987, 32-41).
Lead removal from aqueous streams has largely been based on precipitation by pH adjustment with agents such as CaO. Unfortunately, such methods often result in gelatinous precipitates which are hard to handle, and these methods are usually not effective in solutions containing complex ions. Alternative methods have been proposed including electrodialysis, liquid membrane separation and ion-exchange (Liozidou, M. and R. P. Townsend, `Ion-exchange properties of natural clinoptilolite, ferrierite and mordenite: Part 2. Lead-sodium and lead-ammonium equilibria,` Zeolites, Mar. 1987, 153-159). Application of the natural zeolites mordenite and clinoptilolite to the control of lead pollution has been proposed (Liozidou, M., `Heavy metal removal using natural zeolites,` Proc. 5th Int. Conf. on Heavy Metals in the Environment, Vol. I, 1985, pp 649-651). However, these materials are of relatively low exchange capacity (&lt;2 meq/g) and of unimpressive selectivity, a vital concern in dealing with streams where competing ions predominate.
The aluminum framework enrichment technique employed in the practice of this invention using chabazite may be utilized with other zeolites. Tu, U.S. Pat. No. 4,250,059, describes a technique for preparing a catalytic composite by calcining a zeolite in a mixture with alumina, but does not comment on whether or not the alumina enters the zeolite framework. U.S. Pat. No. 4,683,334 (Bergna, et. al.), relates to modifications of a zeolite which may be chabazite by elements which may be aluminum.
Examples of the use of chabazite in adsorptive or catalytic applications are given by Sherman, et. al. (U.S. Pat. No. 4,663,052), Bergna, et. al. (see above), Coe, et. al. (U.S. Pat. No. 4,732,584) and Abrams, et. al. (U.S. Pat. No. 4,737,592).